1. Field of the Invention
The present invention relates to computer networks and in particular to communication devices by way of which computers are connected together to form such networks.
2. The Prior Art
It is well known to form computer networks comprising a number of computers connected together to enable them to communicate with each other. One well known way of connecting computers together is to provide communication devices having a plurality of ports, to each of which a computing device may be connected. The communication device provides for redistribution of communications received on its ports to the computing devices connected to the other ports.
One particular type of communications device is a repeater. A repeater simply retransmits any communication received on any port to each of its other ports thereby ensuring that each of the computing devices connected to the network receive all communications, enabling them to decide whether the communication is intended for them or not. Other types of communication devices include devices known as bridges which contain some processing capacity so that packets should only be retransmitted on ports necessary to enable those packets to reach their intended destinations. The present invention is applicable to all these types of communication devices but will be described basically in the context of bridges.
In a practical implementation a bridge is generally contained in an enclosure or box which also carries the external connections or ports to which the computing devices forming the network can be connected. For example there may be eight, sixteen or twenty-four ports provided on the device and the bridge is formed in a known manner on a circuit board also including the necessary management processing power to enable the bridge to function as intended.
More recently it has been recognised as desirable to enable the connection of two or more such devices together to increase the number of ports available at a particular location. This is commonly referred to as connecting the devices in a stack or cascade connection of the devices. Advantages of cascade connection of communication devices include the possibility of upgrading existing equipment to provide a greater number of ports without having to discard the existing equipment, and also the provision of more ports in a particular location than may be available on a standard piece of equipment.
Cascade connection between communication devices is typically achieved by providing a cascade or backbone link between the stacked devices. This connection may be made by way of one or two of the ports already provided on the devices for connection to computing devices in the network, alternatively there may be a separate port especially provided for the cascade link. It will be appreciated that, in the basic operation of a repeater, all communications received on any port by one of the devices in the stack will be repeated to the cascade connection in addition to the other ports thereby enabling it to be received by the other devices in the stack and repeated on out of the other ports on those devices. In a bridge arrangement, a received communication is transmitted via the cascade connection if this is necessary to enable the communication to reach its intended destination or destinations.
However, there are other functions which have been provided within a single communications device which present certain difficulties for implementation in a stack. One such feature which is useful to implement in a communication device is xe2x80x9cport mirroringxe2x80x9d. This feature is designed to enable a network manager to monitor traffic passing through a specific port or ports in the network and this may be for any number of reasons, such as to monitor the volume of traffic on a particular port or to monitor the actual activities of a user of a particular port. Port mirroring is typically achieved by connecting a management unit to another port in the network. The communication device on which the port to be monitored is located is configured to send a copy of any communication passing through the port in question also to the management port so that the management device receives that copy.
In a stack of communication devices as discussed above, it may be desired that a management device connected to a port on one box in the stack should be able to monitor activity on a port on another box. In a conventional implementation of this feature, when that other box receives communications via the port in question it not only transmits that communication across the network via the cascade connection according to its normal operation, it additionally sends a copy of that communication to the management port via the cascade connection. This implementation therefore considerably increases the quantity of network traffic carried by the cascade connection which may possibly cause detrimental effect to the operating of the network and the stack in particular.
The present invention provides communications apparatus for a computer network in which a plurality of network devices are enabled to communicate with each other, the apparatus comprising:
a plurality of communication devices each having a plurality of ports via which network communications may be received and transmitted and a communication core means arranged to re-transmit network communications received at said ports according to a defined functionality; and
interconnection means arranged to interconnect said communication devices such that network communications received at each communication device may be transmitted to the or each other communication device;
each said communication device comprising management means, said management means being arranged to have defined therein one or more relationship between a subset of said ports in which communications passing through one of said ports should be communicated to another of said ports, the management means being further arranged to transmit, together with each network communication transmitted to said interconnection means, an indication of whether said network communication has been received at a port on one of said communications devices which has a said relationship with a port in another one of said communication devices, and to read said indications received together with network communications received from said interconnection means.
The invention may be implemented to achieve the above discussed port mirroring function, but may also implement other features such as security features where communications at a particular port are to be diverted.
The present invention is therefore directed to enabling efficient implementation in a stack of communication devices of functions which have previously been implemented within a single communications device, such as those functions discussed above. To achieve this, the present invention uses a protocol of communications on the cascade connection different from that used in the network as a whole. In particular, communications packets sent via the cascade connection have one or more bits added to them, which extra bits convey information about the packet being transmitted from one communication device in the stack to another. On receipt of a communications packet via the cascade connection, a communication device acts on the information provided and will remove the extra bits before retransmission onto the rest of the network to ensure compliance with the overall network protocol.
In the context of port mirroring, the information provided according to the present invention indicates whether the packet in question is also to be transmitted to the management port and this removes the need to transmit the packet in question twice on the cascade.